Reverential, installing last of garden decking (8)
I believe the answer is:
adorning
'decking' is the definition.
(adorn can mean to deck or decorate)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for adorning that I've seen before include "decking" , "Decorating, embellishing" , "Decorating with ornaments" .)